Method for producing light scattering elements

ABSTRACT

The present invention relates to a method for producing light-scattering elements by holographic illumination of a layer ( 10 ) of a photo-sensitive material on a support plate ( 1 ) and subsequent development of the layer ( 10 ) in order to generating a surface structure. The method is distinguished in that the illumination occurs using at least two mutually coherent luminous beams ( 2,3 ) which have passed through one or a plurality of primary diffusers ( 8,9 ) respectively were reflected at one or a plurality of primary diffusers, the luminous beams ( 2,3 ) being irradiated from different directions and being at least partially superimposed on the layer ( 10 ) while forming an interference pattern. This method permits producing diffusers having a leveled scatter profile in a simple manner.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for producinglight-scattering elements or an impression template therefor byholographically illuminating a layer of a photosensitive material on asupport plate and subsequently developing this layer in order togenerate a surface structure.

[0003] Light-scattering elements, such as are produced with the presentmethod, are used, in particular, for visualizing images or sequences ofimages in projection technology. Light-scattering elements in the formof ground glass screens onto which an image is projected are employed inmany image representation systems. The image-generating light impingingon the ground glass screen is scattered by it, thus deflected indifferent directions. This scattering permits recognizing the imageprojected on the ground glass screen from different directions. Forexample, scatter screens or respectively ground glass screens are used,for instance, in 35 mm reflex cameras or in medium-size cameras.Rear-projection or front projection systems also require alight-scattering element in the form of a large-size projection picturescreen or a ground glass screen onto which the image is projected. Thelight field leaving the projection screen respectively the ground glassscreen is diffuse due to the light scattering thus theselight-scattering elements are also called diffusers

[0004] A diffuser can either be operated in transmission, for example inrear projection systems or in reflection, for example in frontprojection systems. In the present patent application, a plate-shapedelement respectively a screen composed of a material that scatters thedirected incident luminous beam by volume scattering or surfacescattering is called a diffuser.

[0005] An essential characteristic of diffusers is their scatter profilewhich shows the angle-resolved scattering efficiency when the incidentluminous beam is collimated perpendicular on the diffuser. Scatteringefficiency is defined as the relative intensity of the scattering beammeasured at a certain angle of emersion normed to the intensity measuredat an angle of emersion respectively an angle of view 0. An entiretwo-dimensional scatter profile comprises the scattering efficiency ofthe entire half space. Usually, a diffuser can be sufficientlycharacterized by two one-dimensional scatter profiles in horizontaldirection, respectively x-direction and in vertical directionrespectively y-direction. A diffuser whose x-scatter profile andy-scatter profile are almost identical is called a symmetrical diffuser.If the x-direction and the y-direction scatter profiles vary greatly,the diffuser is an asymmetrical diffuser.

[0006] The scatter profile of a diffuser must be adapted to the intendedapplication. The geometric conditions and the intended optical effectyield a specific viewing angle range for each application. Thus, forlarge-size projecting screens an asymmetric viewing angle rangerespectively scattering angle range is usually required in which thehorizontal viewing angle should be larger than the vertical viewingangle.

[0007] In the ideal case, the scatter profile of a diffuser comprises arectangular function. In this case, the image inside the specificviewing angle range is always illuminated homogeneously and uniformlybright. On the other hand, no radiation is lost due to scattering in theangle outside the viewing angel range.

[0008] However, the diffusers employed in practice usually do not have arectangular scatter profile but rather a gaussian one. Yet for mostapplications, it is desirable to be able to produce diffusers with aslevel as possible scatter profiles, which in the ideal case would comecloser to a rectangular profile.

[0009] 2. Description of the Prior Art

[0010] There are various prior art methods of producing diffusers. Forinstance diffusers, whose function is based on surface scattering, canbe provided with a scattering surface structure by treating the surfacemechanically, such as for example by sand blasting or by chemicaltreatment of the surface, for example by means of an etching process.However, these methods usually only permit producing symmetricaldiffusers with a gaussian scatter profile. There is no known reliablepossible way of selectively influencing the scatter profile.

[0011] A possible state-of-the art manner of cost-effective massproduction of suited surface structures for the production of diffusersis using moldable materials, such as for example polymers, inconjunction with a replication process. In the replication, using acorrespondingly structured impression template, a surface structure isimpressed on the polymer body yielding the scattering behavior.

[0012] Any desired surface processing method, including the onesdescribed above, can be employed for producing the impression template,which needs to be produced only once. A prerequisite for moldability isthat there is no undercutting in the surface structure.

[0013] Among other things, the impression template can be produced byilluminating a photoresist layer with a scattered light field andsubsequently developing the photoresist layer. The resulting photoresiststructure is then transferred to the impression template, for example bymeans of galvanic molding. The diffusers produced by this method inconjunction with the impression methods reproduce the scatter profile ofthe irradiated (scattered) light field generated, for its part, with theaid of a diffuser, referred to as the primary diffuser hereinafter. Thewidth of the resulting scatter profile depends on the geometry of thesetup. However, the shape of the scatter profile of the produceddiffuser depends on the scatter profile of the primary diffuser and isagain usually almost gaussian.

[0014] State of the art for generating asymmetrical diffusers are, inparticular, volume-holography-based illumination methods with which arefraction index modulation is generated in the volume of alight-sensitive layer. The diffusers produced therewith work accordingto the volume scattering principle.

[0015] U.S. Pat. No. 5,365,354 describes a process for producingasymmetrical diffusers in which a light field is passed through aprimary diffuser and radiated in a light-sensitive material. A volumehologram is recorded by radiation of the light-sensitive material. Withrenewed radiation with light, the volume hologram for its part has ascattering effect due to the reconstruction of the recorded primarydiffuser. In a further embodiment of the method, two diffuse luminousbeams are irradiated from various directions to produce a so-calledmultiplex diffuser. In a multiplex diffuser, diffusion behavior variesin the different directions, because at a certain angle of incidenceonly the hologram of one luminous beam is reconstructed and the hologramof the other luminous beam is reconstructed at another angle ofincidence. Such a type method is only possible by creating volumeholograms and requires that the two wave fields of the two luminousbeams are not mutually coherent. However, the method of U.S. Pat. No.5,365,354 usually leads to gaussian or almost gaussian scatter profiles,because the shape of the scatter profile of the primary diffuseremployed in the illumination setup is duplicated.

[0016] U.S. Pat. No. 3,708,217 also describes a method for recording avolume hologram for producing asymmetrical diffusers. In this method, anobject wave, which was passed through a primary diffuser, issuperimposed with a reference wave on a light-sensitive layer to producea volume hologram. Suited spatial disposal of the primary diffuser inrelation to the light-sensitive layer permits generating a strongdirection-dependent scatter profile. However, the use of a planarreference wave results in the optical transfer function of the volumehologram having a strongly periodic part, which can lead to undesirablecolor effects when employing the diffuser. Furthermore, no solution isoffered in this printed publication with which a level as possiblescatter profile can be realized.

[0017] D. J. Schertler et al's Applied Optics 38 No. 2, 1999, pages291-303 describes a method for generating a leveled scatter profile inwhich a combination of a one-dimensional phase diffraction grid isemployed with a symmetrically scattering diffuser. These two componentscan either be disposed in series as single components or constructed inthe form of the two surfaces of a transparent support material. Thediffuser surface is generated using an etching method and the gridsurface is generated using a photoresist method. Providing such a typediffuser is, however, very complicated and expensive due to thenecessity of producing two components respectively two differentlyshaped surfaces. Furthermore, often undesirable color effects occur whenirradiating white light due to the strictly periodic diffraction grid.

[0018] Based on this state of the art, the object of the presentinvention is to provide a simple method for producing a light-scatteringelement respectively a diffuser with which a leveled scatter profile canbe obtained at least in one dimension.

SUMMARY OF THE INVENTION

[0019] This object is solved using the method according to claim 1.Advantageous preferred embodiments of the method are the subject matterof the subclaims.

[0020] In the present method, a surface structure is generated byholographic illumination of a layer of a photosensitive material on asupport plate and subsequent development of this material. This surfacestructure can be utilized either directly as a diffuser or as thestarting structure for generating a template for producing a diffuser.An element of the present invention is that illumination of the layeroccurs with at least two mutually coherent luminous beams which havepassed through one or through a multiplicity of diffusers respectivelywere reflected at one or at a multiplicity of reflecting diffusers,these luminous beams being irradiated from different directions andbeing at least partially superimposed, forming an interference patternon the layer.

[0021] Preferably, one primary diffuser is positioned in each beam pathof the at least two luminous beams. However, a luminous beam which haspassed through a primary diffuser can also be split into two partialbeams and the photosensitive material can be illuminated therewith.

[0022] In this instance, suited scattering screens having a gaussianscatter profile or another scatter profile can be used as the primarydiffusers. Superimposing the two scattered luminous beams and forming aninterference pattern on the photosensitive layer results, in anadvantageous manner, in a leveling of the scatter profile of thediffuser produced therewith.

[0023] This scatter profile can be influenced by special selection ofthe primary diffusers, by variation of the angle of incidence of theluminous beams or by the use of additional coherent luminous beams,which, if need be, can be also passed through primary diffusers. Ofcourse, further luminous beams which are not coherent to the otherluminous beams can also be utilized to achieve in this manner, if needbe, special scattering effects.

[0024] Depending on the desired scatter profile, both divergent, almostparallel and even convergent luminous beams can be employed forsuperimposition on the light-sensitive layer. In this case, preferably aphotoresist, which has the desired surface relief after developing, isutilized as the light-sensitive layer. This surface relief can then becopied into a support plate, for example composed of glass, locatedunder the surface relief by means of a suited etching technique.Preferably, the surface relief of the photoresist structure is copiedinto a suited material which then serves as the impression template forthe actual generation of diffusers. Molding technology, such as galvanicmolding in nickel, can be employed for this purpose. With the impressiontemplate generated in this manner, the screens made of a impressionablematerial, such as for example a polymer, can then be impressed to obtainthe desired surface structure and therefore the desired scatter profileof the polymer screens. Such type replication techniques are familiar tosomeone skilled in the art so that the present patent application doesnot need to dwell on it.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present method is briefly explained in the following using apreferred embodiment in conjunction with the accompanying drawingswithout the intention of limiting the overall inventive idea.

[0026]FIG. 1 shows a possible setup for generating the surface hologramaccording to the present method;

[0027]FIG. 2 shows a scatter profile of a conventional diffuser havinggaussian distribution;

[0028]FIG. 3 shows two exemplary profiles of diffusers produced with thepresent method;

[0029]FIG. 4 shows greatly simplified a support plate (1) with the layer(10) located thereupon before and after development of the layer (10);

[0030]FIG. 5 shows schematically the transfer of the surface structureof the layer (10) to the support plate (1) by means of an etchingprocess;

[0031]FIG. 6 shows schematically the transfer of the surface structureby means of an impression process to an template (11), which serves asthe impression template for impressing the surface of light-scatteringelements;

[0032]FIG. 7 shows schematically the impression of the surface of platesor foils (12) composed of an impressionable material using theimpression template to generate light-scattering elements.

WAYS TO CARRY OUT THE INVENTION

[0033] The present method utilizes holography technology for producingsurface reliefs by holographically illuminating a suitablephoto-sensitive material on a support plate, hereinafter referred to asa photo plate, the material forming a surface relief in a subsequentetching process in dependence on the illumination in order to produce adiffuser with a leveled scatter profile. The illuminated photo-sensitivematerial is then developed in order to generate a surface relief that isimmediately suited as a diffuser or can be used by means of copying asan impression template for a diffuser. With the method, an interferencefield is generated on the photo-sensitive material by superimposing theat least two mutually coherent luminous beams passed through at leastone primary diffuser respectively reflected at at least one primarydiffuser and converted into a surface modulation by a developing step.

[0034]FIG. 1 shows an example of an arrangement for illuminating the notdiscernable photosensitive layer on a photo plate 1 by superimposing thetwo luminous beams 2 and 3. The two luminous beams 2 and 3 are generatedby a laser 4 with a downstream beam splitter 5 and are directed via abeam deflection element 6 and beam-shaping elements 7, such as forexample lenses, through two primary diffusers 8, 9 onto photo plate 1.Suited illumination times, usually lying in the minute range dependingon the photoresist used, are known to someone skilled in the art. In thepresent example, the two luminous beams 2 and 3 strike the photo plateexpanded.

[0035] After illumination and development of the photoresist layer onthe photo plate 1, the surface structure is transferred onto a nickelplate or nickel foil by means of galvanic molding. The plate then servesas the impression template for the polymer foils respectively polymerplates forming the diffusers after the impression step. In this method,surface structures are usually larger than the wavelengths at which thediffusers are employed.

[0036]FIG. 2 shows an example of a scatter profile of a diffuserproduced using volume holography technology by illumination of a beamthat has passed through a primary diffuser. Such a gaussian scatterprofile occurs with most diffusers depending on the mode of production.

[0037] In contrast thereto, FIG. 3 shows two examples of one-dimensionalscatter profiles of diffusers produced using the present method. Thegraphs distinctly show the leveled course of the angle-dependentscattering efficiency compared to the gaussian profile.

[0038] With the symmetrical diffusers it is usually useful to level theprofile in both directions. However, with asymmetrical diffusers thedesired application may require, for instance when used as a projectionscreen, optimizing only one of the two scatter profiles.

[0039] Leveling of the scatter profile in both dimensions can berealized with the present method, for example, in that illumination iscarried out in several steps and the photo plate is turned between thetwo illuminations.

LIST OF REFERENCES

[0040]1 support plate respectively photo plate

[0041]2,3 coherent luminous beams

[0042]4 laser

[0043]5 beam splitter

[0044]6 beam-deflection elements

[0045]7 beam-shaping elements

[0046]8,9 primary diffusers

[0047]10 layer of a photo-sensitive material

[0048]11 template

[0049]12 plate respectively foil of an impressionable material

What is claimed is:
 1. A method for producing light-scattering elementsor an impression template therefor by holographic illumination of alayer (10) of a photo-sensitive material on a support plate (1) andsubsequent development of said layer (10) in order to generating asurface structure, wherein illumination occurs using at least twomutually coherent luminous beams (2,3) which have passed through one ora plurality of primary diffusers (8,9) respectively were reflected atone or a plurality of primary diffusers, said luminous beams (2,3) beingirradiated from different directions and being at least partiallysuperimposed on said layer (10) while forming an interference pattern.2. A method according to claim 1, wherein a photoresist is utilized assaid photosensitive material.
 3. A method according to claim 1 or 2,wherein the surface structure is transferred to said support plate (1)by means of a subsequent etching process.
 4. A method according to claim1 or 2, wherein the surface structure is transferred to a template (11)by means of a subsequent copying process, said template (11) serving asthe impression template for impressing the surface of light-scatteringelements (12).
 5. A method according to claim 4, wherein the surfacestructure is transferred to a plate or a foil composed of metal by meansof a galvanic molding process.
 6. A method according to claim 4 or 5,wherein the at least one surface of said plates or foils (12) composedof impressionable material respectively a layer of an impressionablematerial applied on a support plate is impressed with the impressiontemplate in order to generate light-scattering elements.
 7. A methodaccording to one of the claims 1 to 6, wherein said at least twomutually coherent luminous beams (2,3) are generated by a laser (4)whose initial beam is split into said at least two luminous beams (2,3).8. A method according to claim 7, wherein a primary diffuser (8,9) ispositioned in the beam path of the initial beam.
 9. A method accordingto claim 7, wherein following splitting of the initial beam at least onesaid primary diffuser (8,9) is positioned in each of the split beampaths of said at least two luminous beams (2,3).
 10. A method accordingto one of the claims 1 to 9, wherein said primary diffusers (8,9) havingan approximately gaussian scatter profile are utilized.
 11. A methodaccording to one of the claims 1 to 10, wherein at least one additionalluminous beam is employed for illuminating said layer (10), saidadditional luminous beam not being coherent to said at least twomutually coherent luminous beams (2,3).
 12. A use of one or a pluralityof diffusers produced using a method according to one or a plurality ofthe preceding claims in a holographic method for producing surfacediffusers.